The present invention relates to a perpendicular magnetic film and a multi-layered film for a perpendicular magnetic film. More particularly, the present invention relates to a perpendicular magnetic film composed of Co-containing .gamma.-Fe.sub.2 O.sub.3 which is suitable as a perpendicular magnetic recording material and which has an excellent oxidation resistance, an excellent corrosion resistance, a large coercive force, a large squareness, a large Faraday rotation angle .theta..sub.F and a small optical absorption coefficient .alpha. and, hence, a large figure of merit, and to a multi-layer film used as a precursor for such a perpendicular magnetic film.
With recent remarkable demand for miniaturization and higher reliability of information processing devices and systems, perpendicular magnetic films as a high-density recording medium have been rapidly developed and put to practical use. A perpendicular magnetic film has an easy magnetization axis perpendicular to the film surface, in other words, perpendicular magnetic anisotropy, and it is required to have excellent coercive force Hc and squareness (residual magnetization .sigma.r/saturation magnetization .sigma.s).
In addition, since the magnetic characteristics of a magnetic recording medium are deteriorated due to oxidation by the oxygen in the air, the perpendicular magnetic film is required to be stable against oxidation.
When a perpendicular magnetic film is used as a medium for magneto-optical recording in which recording and reproduction of information is carried out by using an optical beam such as a laser beam, the medium is required to have excellent magneto-optical characteristics such as a Faraday rotation angle .theta..sub.F, an optical absorption coefficient .alpha. (when the ratio of the intensity I.sub.1 of the light which has transmitted a material having a film thickness of t to the intensity I.sub.0 of incident light, namely, the transmittance (I.sub.1 /I.sub.0) is assumed to be exp (-.alpha.t)) and a figure of merit (2.vertline..theta..sub.F .vertline./.alpha.). With the recent increasing demand for higher-density recording, a growing tendency is shown that the recorded carrier signals have a high frequency, in other words, that the recorded carrier signals have a shorter wavelength. The wavelength of a light source which is used for magneto-optical recording at present is 830 nm or 780 nm, but for the purpose of higher-density recording in the future, semiconductor lasers having a wavelength of 670 nm, 640 nm, 532 nm, 490 nm, etc. are considered as a light source.
In the case of magneto-optical recording, the diameter of a recorded bit is determined by the wavelength of a laser beam, and the shorter the wavelength, the smaller the bit diameter becomes. A material whose Faraday rotation angle has a large dependence on the wavelength in a short wavelength region of not more than 780 nm, is therefore strongly demanded.
An amorphous alloy thin film such as a Gd-Co film and a Tb-Fe film composed of a rare earth metal and a transition metal, a spinel oxide thin film such as a cobalt ferrite film (Japanese Patent Application Laid-Open (KOKAI) Nos. 51-119999 (1976), 63-47359 (1988), 3-17813 (1991), 3-188604 (1991) and 4-10509 (1992)); a magnetoplumbite oxide thin film such as a barium ferrite film (Japanese Patent Application Laid-Open (KOKAI) No. 62-267949 (1987)) and a substitutional garnet thin film have been proposed as perpendicular magnetic oxide thin films for magneto-optical recording.
Among the above-described perpendicular magnetic films, the cobalt ferrite film which is typical of spinel oxides films are stable against oxidation because it is an oxide, it has a high coercive force due to the large crystalline magnetic anisotropy, and it has a large Faraday rotational angle .theta..sub.F in a short wavelength range in the vicinity of 800 nm and 500 nm. Owing to these magnetic characteristics a cobalt ferrite film is considered to be promising as a magneto-optical recording medium.
As the process for producing a cobalt ferrite (CoFe.sub.2 O.sub.4) film, various methods such as a sputtering method, a vacuum evaporation method and a MOCVD method are known. Among these, a sputtering method is mainly adopted. The sputtering method, however, is disadvantageous in that although the easy magnetization axis of a cobalt ferrite (CoFe.sub.2 O.sub.4) film is in the plane (400), the easy magnetization axis is likely to orient at random or the plane (111) is likely to oriented in parallel with the substrate, so that it is difficult to produce a perpendicular magnetic film and it is impossible to obtain a large coercive force and a large squareness. As a method for obtaining a cobalt ferrite (CoFe.sub.2 O.sub.4) film in which the plane (400) is predominantly oriented in parallel with the substrate, (1) the method described in Digits 9th Annual Conf. Magnetics Japan, p315, (2) the method described in Digits 13th Annual Conf. Magnetics Japan, p 246, and (3) the method described in Japanese Patent Application Laid-Open (KOKAI) No. 4-10509 (1992), for example, are known.
The method (1) is a method of depositing Fe and Co ionized in an oxygen plasma on an MgAl.sub.2 O.sub.4 substrate or a silica glass substrate which is heated to a temperature of 500.degree. C. Since it is necessary to maintain the substrate temperature at a high temperature such as not lower than 500.degree. C. during film formation, the productivity is poor. In addition, since the material of the substrate is limited, it is industrially and economically disadvantageous.
The method (2) is a plasma-exciting MO-CVD method. Since it is necessary to maintain the substrate temperature of 300.degree. to 400.degree. C. in a vacuum during film formation, the productivity is poor, which is industrially and economically disadvantageous.
The method (3) is a method of annealing a multi-layered metal film obtained by laminating at least two layers of Co and Fe at a temperature of not lower than 500.degree. C. in an atmosphere containing oxygen. Since a high temperature is necessary, the material of the substrate is limited, which is disadvantageous both industrially and economically.
Although a perpendicular magnetic film which is composed of a spinel oxide and which has an excellent oxidation resistance, an excellent corrosion resistance, a large coercive force, a large squareness, a large Faraday rotational angle .theta..sub.F and a small optical absorption coefficient .alpha., and hence, a large figure of merit, is now in the strongest demand, none of the conventional magnetic thin films sufficiently meet these requirements.
Since the cobalt ferrite (CoFe.sub.2 O.sub.4) film is an oxide, it is stable against oxidation, has a large coercive force due to the crystalline anisotropy which increases with the increase in the Co content, and a large Faraday rotational angle .theta..sub.F in the vicinity of 800 nm and 500 nm. However, the cobalt ferrite (CoFe.sub.2 O.sub.4) film is disadvantageous in that the optical absorption coefficient .alpha. at 700 nm is as large as about 3.5 to 7.0 .mu.m.sup.-1, so that the figure of merit (2.vertline..theta..sub.F .vertline./.alpha.) is small when using a laser beam having a wavelength of not more than 700 nm.
Especially, in order to maintain the substrate temperature at not less then 500.degree. C., the heat resistance of the substrate itself is required, but the heat resistance of polycarbonate, epoxy resin, etc. which are generally used as a material of the substrate is insufficient. Since the material of the substrate is thus limited, it is disadvantageous both industrially and economically.
A magnetic recording medium using an oxide layer as a primary layer has been proposed in Japanese Patent Application Laid-Open (KOKAI) Nos. 3-17813 (1991), 5-12765 (1993) and 5-166167 (1993).
Japanese Patent Application Laid-Open (KOKAI) No. 5-166167 (1993) discloses a fixed magnetic disk obtained by forming a Co-Zn ferrite film having a spinel crystalline structure in which the plane (100) is predominantly oriented in parallel with the substrate on an NiO film having an NaCl-type crystalline structure.
In the fixed magnetic disk, Co ferrite film is formed on an NiO primary layer so as to accelerate crystalline orientation in the plane (100), but the spacing (2.09 .ANG.) of plane (100) of the NiO layer is smaller than the spacing (not less than 2.10 .ANG.) of the plane (400) of the Co.sub.x Zn.sub.y Fe.sub.3-x-y O.sub.4 layer. When the lattice constant of a primary film is larger than that of a Co ferrite film, the perpendicular anisotropy is generally accelerated, while it is reduced when the lattice constant of a primary film is smaller than that of a Co ferrite film, as shown in Japanese Patent Application Laid-Open (KOKAI) Nos. 3-17813 (1991). The fixed magnetic disk disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 5-166167 (1993), therefore, has a small perpendicular anisotropy.
Accordingly, the present invention aims at the industrially and economically advantageous production of a perpendicular magnetic film which is composed of a spinel oxide, and which has an excellent oxidation resistance, an excellent corrosion resistance, a large coercive force, a large squareness, a large Faraday rotational angle .theta..sub.F in a short wavelength region and a small optical absorption coefficient .alpha., and hence, a large figure of merit, at a temperature of not higher than 500.degree. C. and as low as possible.
As a result of various studies undertaken by the present inventors so as to attain the above aims, it has been found that a perpendicular magnetic film comprising a substrate and a Co-containing .gamma.-Fe.sub.2 O.sub.3 film formed on the substrate in which the plane (400) is predominantly oriented in parallel with the substrate, the Co content is 0.01 to 0.32 mol based on 1 mol of Fe, the spacing of the plane (400) is not more than 2.084, and the optical absorption coefficient at 700 nm is not more than 2.5 .mu.m.sup.-1 obtained by annealing at a temperature of 280.degree. to 450.degree. C. a multi-layered film for a perpendicular magnetic film comprising a multiplicity of units formed on the substrate, each unit being composed of an Fe.sub.3 O.sub.4 layer and a CoO layer laminated with one on top of the other, in which the plane (400) is predominantly oriented in parallel with the substrate, one unit has a thickness of not more than 130 .ANG. and the molar ratio of Co to Fe is between 0.01 and 0.32, the perpendicular magnetic film has not more than 2.5 .mu.m.sup.-1 of an the optical absorption coefficient at 700 nm and shows an excellent oxidation resistance, an excellent corrosion resistance, a large coercive force, a large squareness, a large Faraday rotational angle .theta..sub.F and a small optical absorption coefficient .alpha., and hence, a large figure of merit at 700 nm. On the basis of this finding, the present invention has been achieved.